The present invention relates generally to the field of solid state electrochemical devices, and more particularly to substrate, electrode and cell structures for solid state electrochemical devices.
Solid state electrochemical devices are often implemented as cells including two porous electrodes, the anode and the cathode, and a dense solid electrolyte and/or membrane which separates the electrodes. For the purposes of this application, unless otherwise explicit or clear from the context in which it is used, the term xe2x80x9celectrolytexe2x80x9d should be understood to include solid oxide membranes used in electrochemical devices, whether or not potential is applied or developed across them during operation of the device. In many implementations, such as in fuel cells and oxygen and syn gas generators, the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as oxygen ions, or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material (xe2x80x9cMIECxe2x80x9d). In each case, the electrolyte/membrane must be dense and pinhole free (xe2x80x9cgas-tightxe2x80x9d) to prevent mixing of the electrochemical reactants. In all of these devices a lower total internal resistance of the cell improves performance.
The ceramic materials used in conventional solid state electrochemical device implementations can be expensive to manufacture, difficult to maintain (due to their brittleness) and have inherently high electrical resistance. The resistance may be reduced by operating the devices at high temperatures, typically in excess of 900xc2x0 C. However, such high temperature operation has significant drawbacks with regard to the device maintenance and the materials available for incorporation into a device, particularly in the oxidizing environment of an oxygen electrode, for example.
The preparation of solid state electrochemical cells is well known. For example, a typical solid oxide fuel cell (SOFC) is composed of a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (xe2x80x9ccermetxe2x80x9d), in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of a mixed ionically/electronically-conductive (MIEC) metal oxide on the oxidant side of the cell. Electricity is generated through the electrochemical reaction between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen). The contributions of charge transfer step, mass transfer (gas diffusion in porous electrode), and ohmic losses due to electronic and ionic current flow to the total internal resistance of a solid oxide fuel cell device can be significant. Moreover, in typical device designs, a plurality of cells are stacked together and connected by one or more interconnects. Resistive loss attributable to these interconnects can also be significant.
In work reported by de Souza and Visco (de Souza, S.; Visco, S. J.; De Jonghe, L. C. Reduced-temperature solid oxide fuel cell based on YSZ thin-film electrolyte. Journal of the Electrochemical Society, vol.144, (no.3), Electrochem. Soc, March 1997. p.L35-7. 7), a thin film of yttria stabilized zirconia (YSZ) is deposited onto a porous cermet electrode substrate and the green assembly is co-fired to yield a dense YSZ film on a porous cermet electrode. A thin cathode is then deposited onto the bilayer, fired, and the assembly is tested as an SOFC with good results. In work reported by Minh (Minh, N. Q. (Edited by: Dokiya, M.; Yamamoto, O.; Tagawa, H.; Singhal, S. C.) Development of thin-film solid oxide fuel cells for power generation applications. Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), (Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), Proceedings of Fourth International Symposium Solid Oxide Fuel Cells, Yokohama, Japan, Jun. 18-23, 1995.) Pennington, N.J., U.S.A.: Electrochem. Soc, 1995. p.138-45), a similar thin-film SOFC is fabricated by tape calendaring techniques to yield a good performing device. However, these Nixe2x80x94YSZ supported thin-film structures are mechanically weak, and will deteriorate if exposed to air on SOFC cool-down due to the oxidation of Ni to NiO in oxidizing environments. Also, nickel is a relatively expensive material, and to use a thick Nixe2x80x94YSZ substrate as a mechanical support in a solid state electrochemical device will impose large cost penalties.
Solid state electrochemical devices are becoming increasingly important for a variety of applications including energy generation, oxygen separation, hydrogen separation, coal gasification, and selective oxidation of hydrocarbons. These devices are typically based on electrochemical cells with ceramic electrodes and electrolytes and have two basic designs: tubular and planar. Tubular designs have traditionally been more easily implemented than planar designs, and thus have been preferred for commercial applications. However, tubular designs provide less power density than planar designs due to their inherently relatively long current path that results in substantial resistive power loss. Planar designs are theoretically more efficient than tubular designs, but are generally recognized as having significant safety and reliability issues due to the complexity of sealing and manifolding a planar stack.
Thus, solid state electrochemical devices incorporating current implementations of these cell designs are expensive to manufacture and may suffer from safety, reliability, and/or efficiency drawbacks. Some recent attempts have been made to develop SOFCs capable of operating efficiently at lower temperatures and using less expensive materials and production techniques. Plasma spray deposition of molten electrolyte material on porous device substrates has been proposed, however these plasma sprayed layers are still sufficiently thick (reportedly 30-50 microns) to substantially impact electrolyte conductance and therefore device operating temperature.
Accordingly, a way of reducing the materials and manufacturing costs and increasing the reliability of solid state electrochemical devices would be of great benefit and, for example, might allow for the commercialization of such devices previously too expensive, inefficient or unreliable.
In general, the present invention provides low-cost, mechanically strong, highly electronically conductive porous structures for solid-state electrochemical devices, techniques for forming these structures, and devices incorporating the structures. In preferred embodiments, the invention provides a porous electrode designed for high strength and high electronic conductivity (to lower resistive losses in the device due to current collection). Conventional Nixe2x80x94YSZ based SOFCs may be greatly improved by application of the present invention by, for example, casting a thin layer of Nixe2x80x94YSZ on top of a porous high-strength alloy supportxe2x80x94this also substantially lowers the cost of the device by using inexpensive alloy material for mechanical strength as opposed to nickel. Alternatively, alloys known to have good oxidation resistance can be used to form a high-strength air electrode in a solid state electrochemical device. In this embodiment, an alloy such as Inconel 600 is used to make a porous high-strength electrode onto which an electrolyte membrane is co-fired.
The invention provides solid state electrochemical device substrates of novel composition and techniques for forming thin electrode/membrane/electrolyte coatings on the novel or more conventional substrates. In particular, in one embodiment the invention provides techniques for co-firing of a device substrate (often an electrode) with an electrolyte or membrane layer to form densified electrolyte/membrane films 1 to 50 microns thick, preferably 5 to 20 microns thick. In another embodiment, densified electrolyte/membrane films 1 to 50 microns, preferably 5 to 20 microns thick may be formed on a pre-fired substrate by a constrained sintering process. In some cases, the substrate may be a porous non-nickel cermet incorporating one or more of the transition metals Cr, Fe, Cu, and Ag, or alloys thereof.
In one aspect, the present invention provides a method of forming a ceramic coating on a solid state electrochemical device substrate. The method involves providing a solid state electrochemical device substrate, the substrate composed of a porous non-noble transition metal, a porous non-noble transition metal alloy, or porous cermet incorporating one or more of a non-noble non-nickel transition metal and a non-noble transition metal alloy. The substrate may optionally be coated with a material having high electrocatalytic activity for a specific purpose, for example methane reformation, or oxygen or hydrogen ion formation (e.g., Nixe2x80x94YSZ). A coating of a suspension of a ceramic material in a liquid medium is applied to the substrate material, and the coated substrate is fired in an inert or reducing atmosphere.
In another aspect, the invention provides a solid state electrochemical device. The device includes a sintered substrate composed of a porous non-noble transition metal, a porous non-noble transition metal alloy, or a porous cermet incorporating one or more of a non-noble, non-nickel transition metal and a non-noble transition metal alloy, and a sintered coating of a ceramic material on the substrate.
In yet another aspect, the invention provides a composition. The composition is composed a substrate for a solid state electrochemical device composed of a porous cermet incorporating one or more of the transition metals Cr, Fe, Cu and Ag.
In other aspects, the invention provides devices in accordance with the present invention tailored to specific purposes, for example, oxygen generators, gas separators, solid oxide fuel cells and syn gas generators.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.